A method and apparatus for computing a packed sum of absolute differences

ABSTRACT

A method and apparatus that adds each one of multiple elements of a packed data together to produce a result. According to one such a method and apparatus, each of a first set of portions of partial products is produced using a first set of partial product selectors in a multiplier, each of the first set of portions of the partial products being zero. Each of the multiple elements is inserted into one of a second set of portions of the partial products using a second set of partial product selectors, each of the second set of portions of the partial products being aligned. Each of the multiple elements are added together to produce the result including a field having the sum of the multiple elements.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the field of microprocessors; more particularly the present invention relates to a method and apparatus for computing a sum of absolute differences.

[0003] 2. Description of Related Art

[0004] A sum of absolute differences is used in many applications including video applications such as Motion Pictures Expert Group (MPEG) encoding.

[0005] One method of computing a packed sum of absolute differences (PSAD) of packed data A having eight byte elements A₀ . . . A₇ and packed data B having eight byte elements B₀ . . . B₇ is to compute Ai-Bi and Bi-Ai for each value of i from 0 to 7, select the results that are non-negative, and add the non-negative results together. One implementation uses sixteen adders (two adders for each pair of byte elements), eight muxes (to select the non-negative values from each pair of results) and an adder tree to sum the non-negative results.

[0006] As more devices are used, more silicon area is needed in a semiconductor device. Semiconductor devices generally have a cost proportional to the silicon area used. Therefore, it is desirable to reduce the number of devices used to perform the PSAD instruction.

[0007] One method of computing a PSAD with less devices is to use the same device to serially operate on multiple data elements. For example, one adder may compute A₀-B₀ and B₀-A₀ sequentially, another may compute A₁-B₁ and B₁-A₁ sequentially, etc. This reduces the number of adders (silicon area) used, but increases the amount of time required to compute a PSAD.

[0008] What is needed is a method and apparatus to reduce the amount of silicon area required to implement a PSAD instruction without increasing the time required to compute the PSAD.

SUMMARY OF THE INVENTION

[0009] A method and apparatus that adds each one of multiple elements of a packed data together to produce a result is described. According to one such a method and apparatus, each of a first set of portions of partial products is produced using a first set of partial product selectors in a multiplier, each of the first set of portions of the partial products being zero. Each of the multiple elements is inserted into one of a second set of portions of the partial products using a second set of partial product selectors, each of the second set of portions of the partial products being aligned. Each of the multiple elements are added together to produce the result including a field having the sum of the multiple elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 shows a block diagram illustrating an exemplary computer system 100 according to one embodiment of the invention.

[0011]FIG. 2 illustrates one embodiment of a Packed Multiply-Add (PMAD) operation.

[0012]FIG. 3 illustrates one embodiment of a Packed Add (PADD) operation.

[0013]FIG. 4 illustrates one embodiment of a Packed Maximum (PMAX) operation.

[0014]FIG. 5 illustrates a first embodiment of a Packed Sum of Absolute Differences (PSAD) instruction of the present invention.

[0015]FIG. 6 illustrates a second embodiment of the PSAD instruction of the present invention.

[0016]FIG. 7 illustrates a third embodiment of the PSAD instruction of the present invention.

[0017]FIG. 8 illustrates an embodiment of the PABSRC operation of the present invention.

[0018]FIG. 9 illustrates one embodiment of a packed subtract and write carry/packed absolute value and read carry (PSUBWC/PABSRC) arithmetic element of the present invention.

[0019]FIG. 10 illustrates one embodiment of a PSUBWC/PABSRC apparatus of the present invention.

[0020]FIG. 11 illustrates one embodiment of a packed horizontal add (PADDH) apparatus of the present invention.

[0021]FIG. 12 illustrates the alignment of the eight partial products in a Carry Save Adder (CSA) with Carry Lookahead Adder (CLA) tree according to one embodiment.

[0022]FIG. 13 illustrates one embodiment of a PADDH partial products selector of the present invention.

DETAILED DESCRIPTION

[0023] A method and apparatus to reduce the amount of silicon area required to implement a packed sum of absolute differences (PSAD) instruction without increasing the time required to compute the PSAD is disclosed.

[0024] The invention takes advantage of circuitry used to perform other single instruction multiple data (SLMD) operations such that only a relatively small amount of additional circuitry is needed to provide the PSAD instruction. In one embodiment, the PSAD instruction is implemented using two operations to generate a packed data having multiple absolute differences and an operation to sum the multiple absolute differences in the packed data to produce a PSAD.

[0025] One aspect of the invention is the use of the circuitry for a SIMD add operation to generate a packed data having multiple absolute differences by using each one of a set of sign bits to independently select the add or subtract operation for the corresponding packed data element having multiple differences. In one embodiment, when a sign bit indicates the difference in the corresponding packed data element is negative, the packed data element is subtracted from zero to produce the absolute value of the difference. When the sign bit indicates the difference in the corresponding packed data element is non-negative, the packed data element is added to zero to produce the absolute value of the difference.

[0026] Another aspect of the invention is the use of circuitry for a SIMD multiply or multiply-add, for example, to produce a sum of the packed data elements of a packed data by inserting the packed data elements into an adder tree that is used to sum the partial products in the SIMD multiply or SIMD multiply-add. In one embodiment, the packed data has packed data elements that are absolute differences. However, packed data elements containing other values may be summed using this method and apparatus.

[0027] In one embodiment, these two aspects of the invention are combined to produce a PSAD instruction. Alternatively, each aspect of the invention may be used independently with other instructions to perform the PSAD instruction.

[0028] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the described invention. Some of these specific details may not be required to practice the invention. In other instances, well-known structures, interfaces and processes have not been shown in detail in order to avoid obscuring the described invention.

[0029]FIG. 1 shows a block diagram illustrating an exemplary computer system 100 according to one embodiment of the invention. The exemplary computer system 100 includes a processor 105, a storage device 110, and a bus 115. The processor 105 is coupled to the storage device 110 by the bus 115. In addition, a number of user input/output devices, such as a keyboard 120 and a display 125, are also coupled to the bus 115. The processor 105 represents a central processing unit of any type of architecture, such as a complex instruction set computer (CISC), reduced instruction set computer (RISC), very long instruction word (VLIW), or hybrid architecture. In addition, the processor 105 could be implemented on one or more chips. The storage device 110 represents one or more mechanisms for storing data. For example, the storage device 110 may include read only memory (ROM), random access memory (RAM), magnetic disk storage mediums, optical storage mediums, flash memory devices, and/or other machine-readable mediums. The bus 115 represents one or more busses (e.g., peripheral component interconnect (PCI), industry standard architecture (ISA), extended industry standard architecture (EISA), etc.) and bridges (also known as bus controllers). While this embodiment is described in relation to a single processor computer system, the invention could be implemented in a multi-processor computer system.

[0030]FIG. 1 illustrates that the processor 105 includes a decode unit 140, a set of registers 141, an execution unit 142, and an internal bus 143 for executing instructions. Of course, the processor 105 contains additional circuitry, which is not necessary to understanding the invention. The decode unit 140, the set of registers 141 and the execution unit 142 are coupled together by the internal bus 143. The decode unit 140 is used for decoding instructions received by the processor 105 into control signals and/or microcode entry points. In response to these control signals and/or microcode entry points, the execution unit 142 performs the appropriate operations. The decode unit 140 may be implemented using any number of different mechanisms (e.g., a look-up table, a hardware implementation, a PLA, etc.).

[0031] The decode unit 140 is shown including a packed data instruction set 145 for performing operations on packed data. In one embodiment, the packed data instruction set 145 includes a PMAD instruction(s) 150, a PADD instruction(s) 151, a packed subtract instruction(s) (PSUB) 152, a packed subtract with saturate instruction(s) (PSUBS) 153, a packed maximum instruction(s) (PMAX) 154, a packed minimum instruction(s) (PMIN) 155 and a packed sum of absolute differences instruction(s) (PSAD) 160. The operation of each of these instructions is further described herein. In one embodiment of the invention, the processor 105 supports the Pentium® microprocessor instruction set and the packed data instruction set 145. By including the packed data instruction set 145 into a standard microprocessor instruction set, such as the Pentium® microprocessor instruction set, packed data instructions can be easily incorporated into existing software (previously written for the standard microprocessor instruction set). Thus, many multimedia applications may be executed more efficiently by using the full width of a processor's data bus for performing operations on packed data. This eliminates the need to transfer smaller units of data across the processor's data bus to perform one or more operations one data element at a time. Other standard instruction sets, such as the PowerPC™ and the Alpha™ processor instruction sets may also be used in accordance with the described invention. (Pentium® is a registered trademark of Intel Corporation. PowerPC™ is a trademark of IBM, APPLE COMPUTER, and MOTOROLA. Alpha™ is a trademark of Digital Equipment Corporation.) Alternative embodiments of the invention may contain more or less, as well as different, packed data instructions and still utilize the teachings of the invention.

[0032] In one embodiment of the invention, the execution unit 142 operates on data in several different packed (non-scalar) data formats. For example, in one embodiment, the exemplary computer system 100 manipulates 64-bit data groups and the packed data can be in one of three formats: a “packed byte” format, a “packed word” format, or a “packed double-word” (dword) format. Packed data in a packed byte format includes eight separate 8-bit data elements. Packed data in a packed word format includes four separate 16-bit data elements and packed data in a packed dword format includes two separate 32-bit data elements. Examples of particular operations are discussed below with reference to one packed data format. However, the operations apply similarly to any of the packed data formats of the invention.

[0033] In one embodiment, the opcodes of the instructions of the packed data instruction set 145 are encoded differently depending on whether they operate on signed data or unsigned data. In another embodiment, certain instructions only operate on one type of data: either unsigned or signed.

[0034] In the following description, packed byte data D having packed data elements Di, for example, refers to a single data structure D having N byte elements Di where i ranges from 0 to N-1. In one embodiment, a packed byte data D has eight byte elements. Thus, packed byte data D having packed data elements Di includes packed data elements D₀, D₁, D₂, D₃, D₄, D₅, D₆, and D₇. A reference to computing the packed data elements Fi of packed data F from a packed data D and a packed data E where each packed data element Fi is computed as the packed data element Di minus the packed data element Ei is a shorthand for F₀=D₀−E₀, F₁=D₁−E₁, F₂=D₂−E₂ . . . F₇=D₇−E₇.

[0035]FIG. 2 illustrates one embodiment of the PMAD instruction 150. Each packed data element Ai of a packed word data A is multiplied by the corresponding packed data element Bi of a packed word data B to produce doubleword products that are summed by pairs to generate the two packed data elements T₀ and T₁ of a packed dword data T. Thus, T₀ is A₁B₁+A₂B₂ and T1 is A₃B₃+A₄B₄. As illustrated, the packed data elements of packed dword data T are twice as wide as the packed data elements of the packed word data A and the packed word data B.

[0036]FIG. 3 illustrates one embodiment of the PADD instruction 151. Each packed data element Fi of a packed byte data F is the sum of a packed data element Di of a packed byte data D and a packed data element Ei of a packed byte data E. Similarly in the PSUB instruction 152, each packed data element Fi of the packed byte data F is the packed data element Di of the packed byte data D minus the packed data element Ei of the packed byte data E.

[0037] In the PSUBS instruction 153, each packed data element Fi of the packed byte data F is the packed data element Di of the packed byte data D minus the packed data element Ei of the packed byte data E, except that if the result of the subtraction is below a limit (minimum saturation value), the packed data element Fi is set at the minimum saturation value rather than the result of the subtraction.

[0038]FIG. 4 illustrates one embodiment of the PMAX instruction 154. Each packed data element Fi of the packed byte data F is the greater of packed data element Di of the packed byte data D and packed data element Ei of the packed byte data E. Similarly, in the PMIN instruction 155, each packed data element Fi of the packed byte data F is the lesser of packed data element Di of the packed byte data D and packed data element Ei of the packed byte data E.

[0039] In one embodiment, the packed data elements Ai and Bi are unsigned bytes and packed byte data A and packed byte data B have eight packed data elements each. Other embodiments of the PMAD instruction 150, PADD instruction 151, the PSUB instruction 152, and the PSUBS instruction 153, the PMAX instruction 154, and the PMIN instruction 155 may support other packed data types, such as those with different size packed data elements, a different number of packed data elements, and/or signed packed data elements. Furthermore, different rounding and saturation methods may be used.

[0040] In one embodiment, the PMAD instruction 150, PADD instruction 151, the PSUB instruction 152, and the PSUBS instruction 153, the PMAX instruction 154, and the PMIN instruction 155 are implemented as a PMAD operation, a PADD operation, the PSUB operation, and a PSUBS operation, a PMAX operation, and a PMIN operation, respectively. These operations may be performed as part of a microcode sequence to implement the PSAD instruction 160, for example.

[0041] In one embodiment, a PSAD instruction receives packed byte data D having eight packed data elements Di and packed byte data E having eight packed data elements Ei and computes scalar result R according to the formula: $R = {\sum\limits_{i = 0}^{7}\quad {{D_{i} - E_{i}}}}$

[0042]FIG. 5 illustrates a first embodiment of the PSAD instruction 160.

[0043] In step 500, the first operation is a packed subtract and write carry (PSUBWC) operation. For example, in a PSUBWC F←D, E operation, each packed data element Fi of the packed byte data F is computed by subtracting the packed data element Ei of the packed byte data E from the corresponding packed data element Di of the packed byte data D. Each packed data element in the packed byte data D, E, and F represent an unsigned integer. Each carry bit Ci of a set of carry bits C is stored. Each carry bit Ci indicates the sign of the corresponding packed data element Fi.

[0044] In step 510, the second operation is a packed absolute value and read carry (PABSRC) operation. For example, in a PABSRC G←0, F operation, each packed data element Gi of a packed byte data G is computed by adding a packed data element Fi of the packed byte data F to a zero 501 (if the carry bit Ci indicates the corresponding packed data element Fi is non-negative) and subtracting the packed data element Fi from the zero 501 (if the carry bit Ci indicates the corresponding packed data element Fi is negative).

[0045] In one embodiment, step 500 and step 510 are performed as described with reference to FIGS. 9 and 10.

[0046] In step 520, the third operation is a packed add horizontal (PADDH) operation. For example, in a PADDH R←G, 0 operation, a PMAD circuit is used to produce the result RS having a field that represents the sum of all of the packed data elements of packed byte data G as described with reference to FIGS. 11, 12 and 13 below. The PADDH operation is also referred to as a horizontal addition operation.

[0047] These operations may be used to perform other instructions. For example, the PSUBWC and PABSRC operations may be used to perform a packed absolute difference (PAD) instruction that produces a packed byte data having packed data elements that are the absolute differences of the packed data elements Di and Ei. A PSAD instruction may be implemented using the PADDH operation in combination with numerous other methods to produce a PAD. FIGS. 6 and 7 below illustrate two examples. Furthermore, the PADDH operation may be used as a PADDH operation to sum the packed data elements of the packed byte data D where D is a packed byte data other than a PAD. For example, D may be any packed byte data for which a sum of the packed data elements of the packed byte data D is desired.

[0048]FIG. 6 illustrates a second embodiment of the PSAD instruction 160.

[0049] In step 600, the PMAX operation is used. In a PMAX M←D,E instruction, each packed data element Mi of packed byte data M is the maximum value of the packed data element Di of the packed byte data D and the packed data element Ei of the packed byte data E.

[0050] In step 610, the PMIN operation is used. In a PMIN N←D,E operation each packed data element Ni of the packed byte data N is the minimum value of the packed data element Di of the packed byte data D and the packed data element Ei of the packed byte data E.

[0051] In step 620, the PSUB operation is used. In a PSUB G←M,N operation, each packed data element Gi of the packed byte data G is computed by subtracting the packed data element Ni of the packed byte data N from the packed data element Mi of the packed byte data M.

[0052] In step 630, the PADDH operation is used. In the PADDH R←G,0 operation all of the packed data elements of the packed byte data G are summed together to produce a result R as described in step 520 of FIG. 5.

[0053]FIG. 7 illustrates a third embodiment of the PSAD instruction 160.

[0054] In step 700, the PSUBS operation is used. In the PSUBS M←D,E operation each packed data element Mi of the packed byte data M is computed by subtracting a packed data element Ei of the packed byte data E from a packed data element Di of the packed byte data D, saturated at zero. If the result of the subtraction is less than zero, the packed data element Mi is set to zero (saturated). Otherwise, the packed data element Mi is the result of the subtraction.

[0055] In step 710, the PSUBS operation is used. In the PSUBS N←D,E operation, each packed data element Ni of the packed byte data N is computed by subtracting a packed data element Di of the packed byte data D from a packed data element Ei of the packed byte data E, saturated at zero

[0056] In step 720, a bitwise OR operation is used. In the bitwise OR G←M,N operation, each packed data element Gi of the packed byte data G is computed as the bitwise OR of packed byte data M and packed byte data N. In cases where the packed data element Di is greater than the packed data element Ei, the packed data element Mi is the packed data element Di minus the packed data element Ei and the packed data element Ni is zero. The bitwise OR of the packed data element Mi and the packed data element Ni is the packed data element Di minus the packed data element Ei. In cases where the packed data element Di is less than packed data element Ei, the packed data element Mi is zero and the packed data element Ni is the packed data element Ei minus the packed data element Di. The bitwise OR of packed data element Mi and the packed data element Ni is the packed data element Ei minus the packed data element Di. In cases where the packed data element Di is equal to the packed data element Ei, the packed data element Mi and the packed data element Ni are zero. The bitwise OR of the packed data element Mi and the packed data element Ni is zero.

[0057] In step 720, the PADDH operation is used. In the PADDH R←G,0 operation all of the packed data elements of the packed byte data G are summed together to produce a result R as described in step 520 of FIG. 5.

[0058] The examples in FIGS. 6 and 7 include the PADDH operation in combination with other operations. It will be apparent to one skilled in the art that the PADDH operation may be implemented as an instruction and used with the instructions of the packed data set 145, for example.

[0059]FIG. 8 illustrates an embodiment of the PABSRC operation of the present invention.

[0060] In step 820, the packed byte data F and a set of carry bits C (C₀ . . . C₇) are read. In an alternate embodiment, the packed byte data F and a set of sign bits S (S₀ . . . S₇) are read.

[0061] In step 830, a zero is received.

[0062] In step 835, a counter i is set to zero.

[0063] In step 840, a determination is made whether Fi is negative. In one embodiment, the carry bit Ci corresponding to packed data element Fi is compared with one. If the carry bit Ci is equal to one, step 845 is performed. If the carry bit Ci is not equal to one, step 850 is performed. In another embodiment, the sign bit Si corresponding to packed data element Fi is compared with one. If the sign bit Si is equal to one, step 845 is performed. If the sign bit Si is not equal to one, step 850 is performed.

[0064] In step 845, the packed data element Gi of packed byte data G is computed as 0-Fi. Thus the negative value of Fi is negated to produce the absolute value of Fi. Step 855 is then performed.

[0065] In step 850, the packed data element Gi of packed byte data G is set to equal the packed data element Fi. Thus, the non-negative value of Fi is unmodified to produce the absolute value of Fi.

[0066] In step 855, the counter i is incremented by one.

[0067] In step 860, the counter i is tested against the number of packed data elements in a packed byte data. In one embodiment, there are 8 packed data elements in a packed byte data. If the counter i does not equal 8, step 840 is performed. If the counter i equals 8, the PABSRC operation is completed.

[0068]FIG. 8 illustrates each packed data element Gi of packed byte data G being computed serially. However, in the preferred embodiment, each packed data element Gi of packed data G is computed in parallel.

[0069]FIG. 9 illustrates one embodiment of a PSUBWC/PABSRC arithmetic element of the present invention.

[0070] In one embodiment, a PSUBWC/PABSRC arithmetic element described below is used for each of the packed data elements in a packed byte data.

[0071] A PSUBWC/PABSRC arithmetic element 900 comprises an add/subtract logic 910 and a mux 920. The add/subtract logic 910 is coupled to receive a packed data element D₀ of the packed byte data D, a packed data element E₀ of the packed byte data E, and an add/subtract select control 2 (ADDSEL2) signal. The add/subtract logic 910 generate a carry output bit on the C_(output,0) bus and a packed data element F₀ of the packed byte data F on the F₀ bus.

[0072] The mux 920 is coupled to receive the add/subtract control (ADDSEL) signal, and the carry input bit C_(input,0) on the C_(input,0) bus.

[0073] A register 940 is coupled to the C_(output,0) bus and the F₀ bus.

[0074] When performing a PSUBWC operation, the ADDSEL signal indicates a subtract operation. The control signal CNTR1 is asserted to route the ADDSEL signal through the mux 920 to produce the ADDSEL2 signal. The ADDSEL2 signal indicates a subtract operation. The add/subtract logic 910 produces a packed data element F₀ that is computed as the packed data element D₀ minus the packed data element E₀. The packed data element F₀ is stored in a portion of the register 940. The add/subtract logic 910 also produces carry output bit C_(output,0) that is the carry output of the subtraction of packed data element E₀ from the packed data element D₀ and the carry output bit C_(output,0) is stored in a portion of the register 940. The C_(output,0) signal is a one if the result of the subtract operation is negative and a zero is the result of the subtract operation is non-negative.

[0075] When performing a PABSRC operation, the register 940 is read to produce the C_(input,0) signal. The control signal CNTR1 is deasserted to route the C_(input,0) signal through the mux 920 to produce the ADDSEL2 signal. The ADDSEL2 signal indicates an add or subtract operation depending on the value of the C_(input,0) signal. Recall from above, the packed data elements of packed byte data D are set to zero. The add/subtract logic 910 produces the packed data element F₀ that is the sum of the packed data element D₀ (zero) and the packed data element E₀, if the carry input bit C_(input,0) is a zero. The add/subtract logic 910 produces a packed data element F₀ that is the packed data element D₀ (zero) minus the packed data element E₀, if the carry input bit C_(input,0) is a one. The packed data element F₀ is stored in a portion of the register 940.

[0076] In one embodiment, the PSUBWC/PABSRC arithmetic element 900 is the same circuitry used to perform the PADD instruction 151. The mux 920 is added and the C_(output,0) bus is routed to the register 940 and the C_(input,0) bus is routed to the mux 920 to provide for the PSAD instruction 160.

[0077] By saving the carry bits from the PSUBWC operation and using the saved carry bits to control the subsequent PABSRC operation, the same circuitry used to perform the PADD hardware may be used to perform both the PSUBWC and the PABSRC operations with relatively little additional circuitry.

[0078]FIG. 10 illustrates one embodiment of a PSUBWC/PABSRC apparatus of the present invention.

[0079] A PSUBWC/PABSRC apparatus 1090 is coupled to a D bus having packed data elements D₀, D₁, D₂, D₃, D₄, D₅, D₆, and D₇, an E bus having packed data elements E₀, E₁,E₂, E₃, E₄, E₅, E₆, and E₇ and a C_(input) bus having carry input bits C_(input,0), C_(input,1), C_(input,2), C_(input,3), C_(input,4), C_(input,5), C_(input,6), and C_(input,7). The PSUBWC/PABSRC apparatus 1090 is configured to drive a C_(output) bus includes carry output bits C_(output,0), C_(output,1), C_(output,2),C_(output,3), C_(output,4), C_(output,5), C_(output,6), and C_(output,7) and a result (F) bus includes packed data elements F₀, F₁, F₂, F₃, F₄, F₅, F₆, and F₇. A register 1080 is coupled to the C_(output) bus and the F bus.

[0080] The PSUBWC/PABSRC apparatus 1090 includes PSUBWC/PABSRC arithmetic elements coupled to receive packed data elements and carry input bits and generate packed data elements and carry output bits as shown below in Table 1. TABLE 1 Receives Generates PSUBWC/PABSRC Packed Data Packed Data arithmetic element elements C_(input 1) C_(output 1) element 1000 D₀ and E₀ C_(input 0) C_(output 0) F₀ 1010 D₁ and E₁ C_(input 1) C_(output 1) F₁ 1020 D₂ and E₂ C_(input 2) C_(output 2) F₂ 1030 D₃ and E₃ C_(input 3) C_(output 3) F₃ 1040 D₄ and E₄ C_(input 4) C_(output 4) F₄ 1050 D₅ and E₅ C_(input 5) C_(output 5) F₅ 1060 D₆ and E₆ C_(input 6) C_(output 6) F₆ 1070 D₇ and E₇ C_(input 7) C_(output 7) F₇

[0081] In one embodiment, the PSUBWC/PABSRC apparatus 1090 is the same circuitry used to perform the PADD instruction 151. The register 1080 may be an 80-bits floating point register. In this example, when the 64 least significant bits are used to store a 64-bit packed byte data, eight of the sixteen most significant bits are used to store to set of carry bits on the C_(output) bus.

[0082]FIG. 11 illustrates one embodiment of a PADDH apparatus of the present invention. A set of 16×16 multipliers 1100 is coupled to receive a CNTR2 signal, a first operand on a bus 1140 and a second operand on a bus 1141.

[0083] When the CNTR2 signal is deasserted, a PADDH apparatus 1150 performs the PMAD instruction 150. The set of 16×16 multipliers 1100 multiply each packed data element Ai of the packed word data A received on the bus 1140 with the corresponding packed data element Bi of the packed word data B received on the bus 1141 to produce thirty-two 18-bit partial products using radix 4 multiplication. The eight partial products corresponding to the product of A₀ and B₀ and the eight partial products corresponding to the product of A₁ and B₁ (the first sixteen partial products) are produced on a bus 1101. The eight partial products corresponding to the product of A₂ and B₂ and the eight partial products corresponding to the product of A₃ and B₃ (the second sixteen partial products) are produced on a bus 1102.

[0084] In one embodiment, the set of 16×16 multipliers 1100 use multiple partial product selectors employing Booth encoding to generate partial products. Each partial product selector receives a portion of the multiplicand and a portion of the multiplier and generates a portion of a partial product according to well-known methods.

[0085] A carry-save adder (CSA) tree with carry lookahead adder (CLA) 1110 is coupled to receive the first sixteen partial products on the bus 1101 and generate the sum of the first sixteen partial products on a bus 1103. The sum of the first sixteen partial products on the bus 1103 is the sum of the product of A₀ and B₀ and the product of A₁ and B₁. The CSA tree with CLA 1120 is coupled to receive the second sixteen partial products on the bus 1102 and generate the sum of the second sixteen partial products on a bus 1104. The sum of the second sixteen partial products on the bus 1103 the sum of the product of A₂ and B₂ and the product of A₃ and B₃.

[0086] A shifter 1130 is configured to receive the sum on the bus 1103, the sum on the bus 1104, and the CNTR2 signal and generate the packed dword data T on a bus 1105. When the CNTR2 signal is deasserted, the shifter 1130 passes the dword on the bus 1104 onto the most significant dword of the bus 1105 (corresponding to the packed data element T₁) and the dword on the bus 1103 onto the least significant dword of the bus 1105 (corresponding to the packed data element T₀). The data element T₀ is the sum of the product of A₀ and B₀ and the product of A₁ and B₁ and the packed data element T₁ is the sum of the product of A₂ and B₂ and the product of A₃ and B₃. Thus, when the CNTR2 signal is deasserted, the PADDH apparatus 1150 performs the PMAD instruction 150.

[0087] In one embodiment, the PADDH operation receives a packed byte data G on the bus 1140 and a packed byte data Z on the bus 1141 in which packed data Z has the packed data elements set to zero. The CNTR2 signal is asserted.

[0088] When the CNTR2 signal is asserted, certain partial product selectors (PADDH partial product selectors) within the set of 16×16 multipliers 1100 are configured to insert each packed data element Gi into a portion of one of the first sixteen partial products. The four least significant bytes of packed byte data G, G₀, G₁, G₂,and G₃, are produced in portions of four of the first sixteen partial products using the PADDH partial product selectors. These four partial products are four of the eight partial products generated for the product of A₀ and B₀ as described above in the case of performing the PMAD instruction 150. The four most significant bytes of packed byte data G, G₄, G₅, G₆, and G₇, are produced in portions of four of the first sixteen partial products using the PADDH partial product selectors. These four partial products are four of the eight partial products generated for the product of A₁ and B₁ as described above in the case of performing the PMAD instruction 150. The portions of the eight selected partial products of the first sixteen partial products and all the bit positions of the remaining partial products on the bus 1101 and the bus 1102 are generated (using prior art partial product selectors or PADDH partial product selectors, for example) as described in the case of the CNTR2 signal being deasserted. The portions of the eight selected partial products of the first sixteen partial products and all the bit positions of the remaining partial products on the bus 1101 and the bus 1102 evaluate to be zero since the multipliers (packed data elements of the packed byte data Z) are zero. The PADDH partial product selector and its configuration within a CSA tree with CLA is described with reference to FIGS. 12 and 13.

[0089] The CSA tree with CLA 1110 is coupled to receive the first sixteen partial products on the bus 1101 and generate the sum of the first sixteen partial products on the bus 1103. The sum of the first sixteen partial products on the bus 1103 includes the sum all of the packed data elements of the packed data G in a field within the result (see FIG. 12). A CSA tree with CLA 1120 is coupled to receive the second sixteen partial products on the bus 1102 and generate the sum of the second sixteen partial products on the bus 1103. The sum of the second sixteen partial products on the bus 1103 is zero.

[0090] A shifter 1130 is configured to receive a result RS having a least significant dword sum on the bus 1103 and a most significant dword sum on the bus 1104 and generate the result R on the bus 1105. The result R includes a field representing the sum all of the packed data elements of packed byte data G. The shifter 1130 performs a right shift operation on the result RS to produce the result R having the field representing the sum all of the packed data elements of packed byte data G aligned with the least significant bit of the result R. In one embodiment, a right shift of RS by 10 bits is used to generate the result R. Thus, when the CNTR2 signal is asserted and all of the packed data elements of packed byte data Z are set to zero, the PADDH apparatus 1150 performs a PADDH operation.

[0091] In one embodiment, the shifter 1130 is a barrel shifter. In another embodiment, the shifter 1130 is a special purpose shifter configured to pass the input data to the output without shifting the input data (in response to the CNTR2 signal being deasserted for a PMAD instruction 150, for example) or shift the field representing the sum all of the packed data elements of packed byte data G to be aligned with the least significant bit of the output (in response to the CNTR2 signal being asserted for the PADDH operation, for example).

[0092] In one embodiment, the CSA with CLA 1110 and the CSA with CLA 1120 is a CSA adder tree with a CLA at the final stage of the tree. However, it will be apparent to one skilled in the art that other configurations of adder trees may be used to sum multiple partial products and implement the PADDH operation according to the present invention.

[0093]FIG. 12 illustrates the alignment of the first sixteen partial products in the CSA tree with CLA 1110 according to one embodiment.

[0094] A partial product 1201 having bits A00-A17, a partial product 1202 having bits B00-B17, a partial product 1203 having bits C00-C17, a partial product 1204 having bits D00-D17, a partial product 1205 having bits E00-E17, a partial product 1206 having bits F00-F17, a partial product 1207 having bits G00-G17, a partial product 1208 having bits H00-H17, a partial product 1211 having bits I00-I17, a partial product 1212 having bits J00-J17, a partial product 1213 having bits K00-K17, a partial product 1214 having bits L00-L17, a partial product 1215 having bits M00-M17, a partial product 1216 having bits N00-N17, a partial product 1217 having bits O00-O17, and a partial product 1218 having bits P00-P17 are added together in the CSA tree 1210 to produce a result 1200 having bits R00-R31.

[0095] The PADDH partial product selectors are configured to insert the packed data element G₀ at A10-A17, the packed data element G₁ at B08-B15, the packed data element G₂ at C06-C13, the packed data element G₃ at D04-D11, the packed data element G₄ at I10-I17, the packed data element G₅ at J08-J15, the packed data element G₆ at K06-K13, and the packed data element G₇ at L04-L11. The remaining partial product selectors produce bits having the value of the partial products resulting from the multiplication. Since, the multiplier is selected to be zero, all the bits of all the partial products are zero except within a field 1220 and a field 1221 in which the packed data elements of the packed byte data G are inserted.

[0096] In one embodiment, each bit within a field 1222 of the result 1200 is computed as follows. R10 is computed as the sum of A10, B08, C06, D04, E02, F00, I10, J08, K06, L04, M02, and N00. R11 is computed as the sum of A11, B09, C07, D05, E03, F01, I11, J09, K07, L05, M03, N01 and the carry output of the sum of A10, B08, C06, D04, E02, F00, I10, J08, K06, L04, M02, and N00. R12-R19 are computed similarly.

[0097] In one embodiment, the CSA tree with CLA 1120 does not contain PADDH partial product selectors. The sum of the second sixteen partial products is zero since the packed data elements of packed byte data Z (multipliers) are set to zero during the PADDH operation and none of the packed data elements of packed data D are inserted into the second sixteen partial products.

[0098] The shifter 1130 receives the most significant dword of the result RS on the bus 1104 (all zeroes) and the least significant dword of the result RS on the bus 1103. The result RS includes the field 1222 that represents the sum of the packed data elements G₀, G₁, G₂, G₃, G₄, G₅, G₆, and G₇. The shifter 1130 performs a right shift of the result RS by 10 bits to produce the result R having the field that represents the sum of the packed data elements G₀, G₁, G₂, G₃, G₄, G₅, G₆, and G₇ aligned with the least significant bit of the result R. The result R is produced on the bus 1105.

[0099] In an alternate embodiment, the packed data elements of the packed byte data G are added together in the CSA with CLA 1120 and a right shift operation of the result RS by 42 bits is used to generate the result R.

[0100] In yet another embodiment, the packed data elements of packed byte data G are added together in the CSA with CLA 1110 and the packed data elements of packed byte data F are added together in the CSA with CLA 1120 to produce the result RS having a first field (in the most significant dword of the result RS) containing the sum of the packed data elements of packed byte data G and a second field (in the least significant dword of the result RS) containing the sum of the packed data elements of packed byte data F. A right shift operation on the result RS may be used to align each field with the least significant bit of the corresponding dword of the result R.

[0101] It will be apparent to one skilled in the art that the packed data elements may be inserted in numerous locations depending on factors such as the number of packed data elements to be inserted, the size of the packed data elements to be inserted, the size of the partial products and the number of the partial products. For example, four additional packed data elements may be inserted at bits E02-E09 of the partial product 1205, bits F00-F07 of the partial product 1206, bits M02-M09 of the partial product 1215, and bits N00-N07 of the partial product 1216 to be summed with the eight packed data elements in the field 1220 and the field 1221 as illustrated in FIG. 12. In another example, the eight packed data elements may be inserted at bits B13-B06 of the partial product 1202, bits C11-C04 of the partial product 1203, bits D09-D02 of the partial product 1204, and bits E07-E00 of the partial product 1205, bits J13-J06 of the partial product 1212, bits K11-K04 of the partial product 1213, bits L09-L02 of the partial product 1214, and bits M07-M00 of the partial product 1215. The result RS has the field 1222 at bits R08-R17 of the result 1200.

[0102]FIG. 13 illustrates one embodiment of a PADDH partial product selector of the present invention.

[0103] The following example illustrates a partial product selector used to generate bit B10 of the partial product 1202 and insert bit 2 of a packed data element G₁ of a packed byte data G at bit B 10 of the partial product 1202 when performing a PADDH operation.

[0104] A PADDH partial product selector 1300 is coupled to receive an MB₁₀ signal, a G_(1,2) signal, and a SELECT signal, and generate an O_(1,B10) signal. The SELECT signal includes a booth encoding (BOOTH) control signal and an additional control (CTRL2) signal. The M_(B10) signal represents the booth encoded multiplicand bits corresponding bit B10 of the partial product 1202. The BOOTH signal represents a portion of the multiplier. The G_(1,2) signal represents bit 2 of a packed data element G₁ of a packed data G for the PADDH operation.

[0105] When the CTRL2 signal is not asserted, the PADDH partial product selector 1300 receives the the M_(B10) signal and produces bit B10 of the partial product 1202 by selecting one of the bits of the MB₁₀ signal to be driven on the O_(1,2) signal according to the well-known booth encoding method.

[0106] When the CTRL2 signal is asserted, the PADDH partial product selector 1300 receives the G_(1,2) signal and drives the O_(1,2) signal to the same value to insert bit 2 of the packed data element G₁ of the packed data G into bit B10 of the partial product 1202. By using multiple PADDH partial product selectors, all the bits of the packed data element G₁ may be inserted into the partial product 1202. Furthermore, other packed data elements may be inserted into other partial products using more PADDH partial product selectors.

[0107] In one embodiment, the PADDH partial product selector 1300 is a standard partial product selector (the BOOTH signal) with an extra control signal (the CNTR2 signal) to select between the standard input (the M_(B10) signal) and an extra input (the G_(1,2) signal). Thus, there is little incremental logic needed to implement the PADDH operation. 

What is claimed is:
 1. A computer implemented method of adding each of a first plurality of elements of a first packed data together to produce a first result comprising the steps of: producing a first plurality of portions of a plurality of partial products using a first plurality of partial product selectors in a multiplier, each of the first plurality of portions of the plurality of partial products being zero; and inserting each of the first plurality of elements into one of a second plurality of portions of the plurality of partial products using a second portion of a plurality of partial product selectors, each of the second plurality of portions of the plurality of partial products being aligned; and adding each of the first plurality of elements together to produce the first result including a field having the sum of the first plurality of elements.
 2. The computer implemented method of claim 1 further comprising the step of shifting the first result to produce a second result having the field aligned with the least significant bit of the second result.
 3. The computer implemented method of claim 1 wherein the first plurality of elements each consist of eight elements, each of the first plurality of elements being an unsigned byte.
 4. The computer implemented method of claim 1 wherein the multiplier has zero as one operand.
 5. The computer implemented method of claim 1 further comprising the steps of: producing a fourth packed data having a fourth plurality of elements and a plurality of sign bits, each of the fourth plurality of elements and the plurality of sign bits being computed by subtracting one of a second plurality of elements of a second packed data from a corresponding one of a third plurality of elements of a third packed data; and producing the first packed data, each of the first plurality of elements being computed by subtracting one of the fourth plurality of elements from the corresponding one of an at least one element, if the corresponding one of the plurality of sign bits is in a first state; and adding one of the fourth plurality of elements from the corresponding one of the at least one element, if the corresponding one of the plurality of sign bits is in a second state.
 6. The computer implemented method of claim 1 further comprising the steps of: producing a fourth packed data having a fourth plurality of elements, each of the fourth plurality of elements being maximum value of one of a second plurality of elements of a second packed data and one of a third plurality of elements of a third packed data; producing a fifth packed data having a fifth plurality of elements, each of the fifth plurality of elements being minimum value of one of the second plurality of elements and one of the third plurality of elements; and producing the first packed data, each of the first plurality of elements being one of the fifth plurality of elements minus corresponding one of the fourth plurality of elements.
 7. The computer implemented method of claim 1 further comprising the steps of: producing a fourth packed data having a fourth plurality of elements each of the fourth plurality of elements being one of a second plurality of elements of a second packed data minus the corresponding one of a third plurality of elements of a third packed data saturated at zero; producing a fifth packed data having a fifth plurality of elements, each of the fifth plurality of elements being one of the third plurality of elements minus the corresponding one of the second plurality of elements saturated at zero; producing the first packed data, each of the first plurality of elements being a bitwise logical OR of one of the fourth plurality of elements and the corresponding one of the fifth plurality of elements.
 8. An apparatus for performing a horizontal add of a first packed data having a first plurality of elements comprising: a multiplier operable to produce a first plurality of portions of a plurality of partial products using a first plurality of partial product selectors, each of the first plurality of portions of the plurality of partial products being zero and inserting each of the first plurality of elements into one of a second plurality of portions of the plurality of partial products using a second plurality of partial product selectors, each of the second plurality of portions of the plurality of partial products being aligned; and an adder tree configured to receive the plurality of partial products and produce a first result including a field having the sum of the first plurality of elements.
 9. The apparatus of claim 8 wherein the first plurality of partial product selectors are coupled to receive a portion of an operand, the operand being zero.
 10. The apparatus of claim 9 wherein the operand has a single element.
 11. The apparatus of claim 9 wherein the operand is a packed data.
 12. The apparatus of claim 8 wherein the adder tree comprises: a plurality of carry save adders operable to receive the plurality of partial products and produce a set of carry and sum signals; and a carry lookahead adder operable to receive the set of carry and sum signals and generate the first result, the first result being the sum of the set of carry and sum signals.
 13. The apparatus of claim 8 further comprising a shifter coupled to receive the first result and shift the first result to produce a second result having the field aligned with the least significant bit of the second result.
 14. The apparatus of claim 8 further comprising: a first circuit configured to receive a second packed data having a second plurality of elements and a third packed data having a third plurality of elements to produce a fourth packed data having a fourth plurality of elements, each of the fourth plurality of elements being maximum value of one of the second plurality of elements and one of the third plurality of elements; a second circuit configured to receive the second packed data and the third packed data to produce a fifth packed data having a fifth plurality of elements, each of the fifth plurality of elements being minimum value of one of the second plurality of elements and one of the third plurality of elements; and a third circuit configured to receive the fourth packed data and the fifth packed data to produce the first packed data, each of the first plurality of elements being one of the fifth plurality of elements minus corresponding one of the fourth plurality of elements.
 15. The apparatus of claim 8 further comprising: a first circuit configured to receive a second packed data having a second plurality of elements and a third packed data having a third plurality of elements to produce a fourth packed data having a fourth plurality of elements, each of the fourth plurality of elements being one of the second plurality of elements minus the corresponding one of the third plurality of elements saturated at zero; a second circuit configured to receive the second packed data and the third packed data to produce a fifth packed data having a fifth plurality of elements, each of the fifth plurality of elements being one of the third plurality of elements minus the corresponding one of the second plurality of elements saturated at zero; and a third circuit configured to receive the fourth packed data and the fifth packed data to produce the first packed data, each of the first plurality of elements being a bitwise logical OR of one of the fourth plurality of elements and the corresponding one of the fifth plurality of elements. 